Aug. 09, 2011

With its handsome black-and-white coloration, Commerson's dolphin is a striking addition to the cool subantartic waters off southern South America and surrounding the Kerguelen Islands in the southern Indian Ocean. Its distinctive appearance is unmistakable: black head and flippers, white throat patch and body, and wide black band extending from just in front of the dorsal fin to the caudal region, which along with the tail is black all around.

Commerson's dolphin is named for French naturalist Philibert Commerson, who sighted the animal in the Strait of Magellan and near Tierra del Feugo in 1767 during a voyage to circumnavigate the globe. He wrote of his observations in a letter addressed to fellow French naturalist Georges-Louis Leclerc, count de Buffon. The letter eventually fell into the hands of Étienne de La Ville-sur-Illon, count de Lacépède, a professor of zoology at the Museum of Natural History in Paris, who in 1804 gave the dolphin its scientific name, Cephalorhynchus commersonii, in honor of Commerson.

Commerson's dolphin is relatively small, with an average weight of around 190 pounds and with individuals near South America measuring about 57 or 58 inches in length and those near the Kerguelen Islands measuring about 66 to 69 inches in length (females of both populations are slightly larger than males). For comparison, the common bottlenose dolphin (Tursiops truncatus) weighs between 570 and 1,100 pounds and measures from 94 to 150 inches in length (males are heavier and longer than females).

The Commerson's dolphin population off the coast of the Kerguelen Islands was discovered in the 1950s, and in addition to being larger than its counterpart living off the coast of southern South America, there are other differences between the two. For example, the Kerguelen dolphins have a grayer body color than the South American dolphins, which usually have bright white bodies. The former also are adapted to water temperatures that are slightly cooler, ranging from about 1 to 8 °C, than the temperatures known to the population near South America, which enjoys water ranging from about 4 to 16 °C.

Genetic analyses have indicated that the two populations are in fact distinct, and each is considered a subspecies: C. c. kerguelenensis representing the Kerguelen individuals and C. c. commersonii the Commerson's dolphins found in the Strait of Magellen and near Tierra del Fuego, Patagonia, and the Falkland Islands. The two subspecies have not mixed probably for the last 10,000 years, when the Kerguelen population is thought to have been established by individuals from South America who swam east from their home waters, perhaps having followed the Antarctic Circumpolar Current (the West Wind Drift). In 2006 researchers reported the sighting of a South American Commerson's dolphin in waters near South Africa, nearly 2,500 miles beyond its known range and on the circumpolar track to the Kerguelen Islands, lending support to the theory about how the population in the southern Indian Ocean became established so long ago.

While the Kerguelen population feeds primarily on fish, the South American dolphins feed on a variety of prey, including small fish, squid, octopus, shrimp, tunicates, and marine worms. Both subspecies may hunt in groups, driving fish such as sardines and silversides together into a concentrated pack or close to the shoreline, making them easier to catch. They also spend time in kelp beds and areas with a sweeping tidal range, where wave action stirs up prey buried beneath the mud.

The isolation of the Kerguelen population renders the dolphins there naturally susceptible to threats such as climate change and global warming. The Commerson's dolphins around southern South America, however, have much more to worry about. For example, they are often caught in fishing nets in coastal areas and sometimes are even trapped in trawl nets used for catching shrimp. Studies conducted in the early 2000s indicated that more than 350 Commerson's dolphins may be caught as by-catch each year near Chubut and Santa Cruz (Argentinian provinces that are part of Patagonia). Because data on total population size is deficient, however, the mortality rate from human-related factors for Commerson's dolphin is not known. So until more population information becomes available, and until fishery regulations are improved, we can only hope that they exist in large numbers.
____________Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.

About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.